1. Field of the Invention
The present invention relates generally to a plasma treatment system suitable for a treatment, such as CVD, etching, sputtering or ashing.
2. Description of the Prior Art
In recent years, with the densification and high definition of semiconductor products, a plasma treatment system for performing a treatment, such as thin-film deposition, etching or ashing, is used in a process for producing semiconductor products.
Conventionally, as plasma treatment systems of this type, there are known a system wherein a microwave inlet is provided in a plasma producing chamber having a magnetic field forming means to form an electron cyclotron resonance cavity to draw ions from the plasma producing chamber to irradiate a semiconductor wafer with ion beams in a reaction chamber (Japanese Patent Publication No. 58-13626), and a system for introducing microwaves into a cavity resonator from one end thereof through a waveguide to radiate microwaves into a plasma producing chamber from a slot provided at the other end of the resonator (Japanese Patent Laid-Open No. 63-293825).
However, since the system disclosed in Japanese Patent Publication No. 58-13626 has the plasma producing chamber and the reaction chamber, there is a problem in that the size of the whole system is great. In addition, since the system disclosed in Japanese Patent Laid-Open No. 63-293825 has the cavity resonator and the plasma producing chamber, there is the same problem.
On the other hand, in a plasma treatment system wherein a circular microwave radiating plate member (a flat antenna member) having spirally or concentrically arranged slots is mounted on the tip of a coaxial guide to be arranged in a vacuum vessel (a treatment vessel) having a discharge space and wherein a sample, such as a semiconductor wafer, arranged in the vacuum vessel so as to face the microwave radiating plate member is irradiated with microwaves (Japanese Patent Laid-open No. 1-184923 and 8-111297), it is possible to miniaturize the whole system since the vacuum vessel itself has a cavity resonator structure.
However, in the above described system disclosed in Japanese Patent No. 8-111297, there is a problem in that it is not possible to input an electric power higher than some extent even if a great electric power is intended to be input in order to obtain a high density plasma.
Therefore, the inventors have proposed a plasma treatment system for solving this problem in Japanese Patent Application No. 8-153357 (Japanese Patent Laid-Open No. 9-181052). In this plasma treatment system, the length of each of slots formed in a flat antenna member and the space between adjacent two of the slots are different from those of the above described system disclosed in Japanese Patent Laid-Open No. 8-111297, so that an electric field exponentially attenuating as leaving the surface of the flat antenna member can be formed in a treatment space in a treatment vessel. The radial space between adjacent two of the slots of the flat antenna member is set to be preferably in the range of from 5% to 50% of the coaxial guide wavelength of microwaves (substantially the same value as the coaxial guide wavelength in the case of the plasma treatment system disclosed in Japanese Patent Laid-Open No. 8-111297), and the length of each of the slots is set to be preferably in the range of (1/2).+-.30% of the coaxial guide wavelength (in the range of from about 1/2 of the coaxial guide wavelength to about 1/2 of a free space wavelength in the case of the plasma treatment system disclosed in Japanese Patent Laid-Open No. 8-111297).
According to the plasma treatment system with this construction, since an electric power for producing plasma is inputted via an electrostatic field, not via an electromagnetic field, there is no upper limit to the input electric power, so that it is possible to efficiently the electric power.
However, it was revealed that, in this plasma treatment system, there is a problem in that the intensity of the electric field in the treatment space facing the central portion of the flat antenna member increases. The inventors consider that the reason for this is as follows. The microwaves supplied from the inner conductor of the coaxial waveguide to the center of the flat antenna member propagate in radial directions while being radiated into the treatment space via the slots. Since the space between adjacent two of the slots is set to be the above described value in this plasma treatment system, a very small amount of microwaves are radiated radially outside via the slots. Therefore, most of the microwaves reflect on a radial edge (the inner wall surface of a radial waveguide box of a conductive material) to be returned. Then, the returned microwaves are radiated radially inside via the slots. Since the normal line of each of the slots formed-in the flat antenna member passes through the center of the flat antenna member, the microwaves radiated from the slots concentrate on the treatment space facing the central portion of the flat antenna member, and the intensity of the electric field herein increases.
In addition, in the above described plasma treatment system proposed in Japanese Patent Laid-Open No. 9-181052, it was revealed that the density of plasma directly beneath a quartz glass disposed on the bottom surface of the flat antenna member, i.e., the density of plasma in a plasma forming region, is ununiform. It is considered that the reason for this is that, if the slot pattern formed in the flat antenna member is axisymmetric, the axisymmetric coaxial waveguide resonates with the axisymmetric flat antenna member in an inherent mode. Furthermore, the expression "axisymmetric" means that, like a concentric circle, the shape of the coaxial waveguide or flat antenna member does not change even if it is rotated about the center of the flat antenna member by an optional angle.